Electronic equipment assemblies such as, for example, telecommunications equipment, generally comprise a cabinet that houses a number of printed circuit card assemblies within one or more shelves. Each printed circuit card assembly includes a printed circuit board with electronic components mounted thereon. In order to reduce equipment size, manufacturers must make maximum use of the space available on the printed circuit boards of the printed circuit card assemblies that comprise the electronic equipment assembly by increasing component density on each printed circuit board. Because a majority of the components found on the printed circuit boards are made of plastic or similar combustible material, the fuel load carried by each printed circuit card assembly within an electronic equipment assembly has increased tremendously. Moreover, these component-dense printed circuit card assemblies are inserted into the shelves of the electronic equipment assemblies at a tight card-to-card pitch in order to further reduce the size of the electronic equipment assembly. This decreased spacing between printed circuit card assemblies allows for an increased number of card assemblies per shelf/system and thus increases the amount of heat generated by the shelf/system, thereby increasing the risk of fire, and increasing the likelihood of adjacent printed circuit card assemblies igniting each other should one printed circuit card assembly catch fire.
Accordingly, flame spread through an electronic equipment assembly has become of great concern given these increased fuel loads resulting from increased component density on the printed circuit card assembly and increased printed circuit card assembly density within the shelves of the equipment.
Consider, for example, a telecommunications switching point that controls a regional calling area that might contain several electronic equipment assemblies. Should one printed circuit card assembly of one electronic equipment assembly catch fire, and that fire spreads unchecked through the electronic equipment assembly, the fire is likely to spread beyond that assembly to other electronic equipment assemblies resulting in loss of millions of dollars in telecommunications equipment, loss of telephone service for that area, as well as possible loss of life. As a result, guidelines that permit manufacturers to sell such electronic equipment assemblies have become increasingly more stringent.
In particular, GR-63-CORE, issued Oct. 1, 1995, sets forth the Telcordia (previously Bellcore)/NEBS standard which requires that all equipment assemblies pass a flame spread test based on the American National Standards Institute (ANSI) standard for fire propagation hazard testing in telecommunication equipment. The test requires that a burner be ignited and then placed within the electronic equipment assembly.
Specifically, sections 4.2.2 and 5.2.3 of GR-63-CORE, issued Oct. 1, 1995, call for a programmable methane line burner to be ignited and inserted into the equipment assembly at a location where it is anticipated that fire spread is most likely to occur. The burner is inserted into the vacated space created by removing one printed circuit card assembly from the shelf under test. The burner is ignited and a flow of methane is held at one liter per minute (l/m) for ten seconds. The flow is increased linearly to nine l/m during the next eighty seconds of the test at which point the flow is decreased linearly back to zero point zero one l/m during the following two-hundred-forty seconds. At this point, three-hundred-thirty seconds into the test, the burner is turned off and observations are made and recorded as to whether or not the fire is self-sustaining and whether or not there is a continued presence of observable smoke. Smoke, gas, temperature and heat release measurements are recorded throughout the test.
During the first ninety seconds of the GR-63-CORE test, a significant amount of heat is released in the burn area of the printed circuit board of the printed circuit card assembly, that is, the area on the printed circuit board that is most likely to catch fire. The principal modes of heat transfer are radiation and convection. Unprotected boards typically result in the components igniting during this first ninety seconds of the test. Moreover, these components can burn for up to and even beyond five and one half minutes of methane flow.
The current solution to meeting the flame spread test given the current, component-dense printed circuit card assemblies is to increase the printed circuit card assembly pitch in the electronic equipment assembly to provide more space between adjacent printed circuit card assemblies, or else to decrease the fuel load on the printed circuit card assemblies by reducing the number of components per printed circuit board (which requires more printed circuit card assemblies per electronic equipment assembly). Both of these solutions, however, result in undesirable increases in equipment assembly size.